This invention relates to servo-positioning, particularly to the closed-loop servo-positioning techniques used in digital data recording and reproduction, particularly in disc-type record media, where positioning signals are recorded on the disc media for tracking purposes, as for example in systems of the type known as "embedded servo" positioning systems, where the positioning signals are in effect interspersed with the user data fields.
With the ever-increasing desires for achieving both greater quantities of data storage on record media, particularly discs, and for achieving components of smaller and smaller physical size, the recording tracks on record media continually become narrower and narrower, and packed more and more closely together, track densities greater than 500 per inch becoming ever more frequently discussed.
With such high track densities, it becomes mandatory to servo-control the transducer head so that it "locks on" to the desired track and "follows" such track over its actual configuration, rather than being held at a fixed concentric position relative to the disc and making the assumption, well known to be untrue, that the recording tracks are in fact perfectly circular and concentric with the disc.
In such servo-positioning systems, the disc media is frequently pre-recorded with marking signals disposed along the various recording tracks, which can be detected and utilized as servo loop control signals, such pre-recorded signals often being placed between successive user data fields and identified by the expressive term "embedded servo" signals.
Perhaps the most frequently-encountered systems for utilizing embedded servo tracking methods are based upon the principle that there will be analog differences, particularly amplitude differences, between signals transduced from recorded bursts which are disposed in a symmetrical pattern with respect to the recording track centerline if the transducer head is not then accurately aligned over that centerline. Such amplitude (or other analog) differences thus provide the basis for an error signal, which can be used to indicate the magnitude and direction of position changes which will bring the transducer into alignment with the center of the particular track involved.
In processing such signal differences, however, it is necessary to employ normalizing techniques, since the absolute magnitude of the differences will vary in a number of ways, for a number of reasons. Generally speaking, it is believed that the most typical approach heretofore followed in normalizing such signals was to sum the absolute magnitudes thereof and use the summation as an AGC feedback signal to an amplifier feeding the signal detectors, the output of such feedback-compensated pre-amplified detectors thus ostensibly producing the desired normalized signal. However, a number of difficulties arise in such a system, primarily because of variations in the absolute magnitude of the signals, and thus of the signal differences, leading to the introduction of very undesirable error sources and instabilities in the resulting servo-positioning.